A Treatise of Human Nature (1739-40) is a book by Scottish philosopher David Hume, considered by many to be Hume's most important work and one of the most influential works in the history of philosophy. The Treatise is a classic statement of philosophical empiricism, skepticism, and naturalism. In the introduction Hume presents the idea of placing all science and philosophy on a novel foundation: namely, an empirical investigation into human nature. Impressed by Isaac Newton's achievements in the physical sciences, Hume sought to introduce the same experimental method of reasoning into the study of human psychology, with the aim of discovering the "extent and force of human understanding". Against the philosophical rationalists, Hume argues that passion rather than reason governs human behaviour. He introduces the famous problem of induction, arguing that inductive reasoning and our beliefs regarding cause and effect cannot be justified by reason; instead, our faith in induction and causation is the result of mental habit and custom. Hume defends a sentimentalist account of morality, arguing that ethics is based on sentiment and passion rather than reason, and famously declaring that "reason is, and ought only to be the slave to the passions". Hume also offers a skeptical theory of personal identity and a compatibilist account of free will.

Contemporary philosophers have written of Hume that "no man has influenced the history of philosophy to a deeper or more disturbing degree", [2] and that Hume's Treatise is "the founding document of cognitive science"[3] and the "most important philosophical work written in English." However, the public in Britain at the time did not agree, nor in the end did Hume himself agree, reworking the material in An Enquiry Concerning Human Understanding (1748) and An Enquiry Concerning the Principles of Morals (1751). In the Author's introduction to the former, Hume wrote:

"Most of the principles, and reasonings, contained in this volume, were published in a work in three volumes, called A Treatise of Human Nature: a work which the Author had projected before he left College, and which he wrote and published not long after. But not finding it successful, he was sensible of his error in going to the press too early, and he cast the whole anew in the following pieces, where some negligences in his former reasoning and more in the expression, are, he hopes, corrected. Yet several writers who have honoured the Author's Philosophy with answers, have taken care to direct all their batteries against that juvenile work, which the author never acknowledged, and have affected to triumph in any advantages, which, they imagined, they had obtained over it: A practice very contrary to all rules of candour and fair-dealing, and a strong instance of those polemical artifices which a bigotted zeal thinks itself authorized to employ. Henceforth, the Author desires, that the following Pieces may alone be regarded as containing his philosophical sentiments and principles."